X-ray diffraction is a well known technique in which an X-ray beam is directed at a sample and the diffracted, or scattered, X-rays are measured to obtain information about the sample. The diffraction angle is related to the length scale probed by the measurements—larger length scales correspond to smaller scattering angles.
Small angle X-ray scattering is a measurement technique that is used for structural investigation in the nm range. An X-ray beam is directed at a sample, and the intensity as a function of scattering angles is measured for small scattering angles, typically less than 7°. At these small angles, the X-ray beam can probe structure in the sample at a slightly larger length scale than the sub-nm range probed by conventional, wide angle X-ray diffraction.
Unlike conventional X-ray diffraction (XRD) techniques, the sample for small angle scattering need not be crystalline, and so the technique may be used for determining properties of macromolecules and partially ordered structures for which X-ray diffraction is not suitable. The sample need not be in solid form and the technique can be used on liquids, solutions and suspensions, as well as solids.
Most applications of SAXS require vacuum isolation to prevent air scattering.
Typically, SAXS equipment consists of a vacuum housing, which is fixed to a separate X-ray source. The housing contains a sample holding portion, various X-ray alignment components including in particular some beam conditioners to restrict the incident beam trajectories and a beam stop to absorb X-rays that are not scattered at all. X-ray detector may be based inside or outside the housing.
The SAXS technique is not as widely used as might be expected, but this may well be due to the required dedicated equipment. The use of conventional XRD equipment to detect small angle scattered radiation has not given the best quality results
Wide angle X-ray scattering (WAXS) is a similar approach at larger angles. WAXS may target the scattering from periodic patterns of electron density or the scattering from isolated particles, measurements complimentary to SAXS.